In the landing, take-off and, more generally, ground movement phases of an aircraft, knowledge of the state of the runway surface is of paramount importance.
This surface state, or “runway state”, has been standardized in a scale of runway states providing a plurality of discrete values: generally, dry runway (DRY), wet/damp runway (WET), soaked runway (WATER), runway with compacted snow (CSNW), snowy runway (SNW), frozen runway (ICE), etc.
Specifically, predicting the braking performance of the aircraft depends on this knowledge. It is thus possible:                to best estimate the distance required to stop the aircraft during its landing, for the sake of safety;        not to overestimate this stopping distance required to bring the aircraft to a halt and therefore not to have, unduly, a negative impact on the usage operations of the runway and the aircraft.        
Numerous pilot assist systems require precise knowledge of this runway state.
For example, documents FR2817979 and FR2857468 propose devices for assisting piloting in the approach and landing phases, known as “Brake to Vacate” (BTV), allowing the braking of the aircraft to be monitored and controlled via closed-loop control laws. These control laws depend directly on the estimation of stopping distances on the basis of the runway state.
On the other hand, documents FR2936077 and FR2914097 propose devices for assisting piloting in the approach and landing phases, known as “Runway Overrun Protection” (ROP) or “Runway Overrun Warning” (ROW), making it possible to detect a risk of overrunning the runway depending on the runway state, in order to warn the pilot either to execute a go-around or to fully apply the brakes.
However, the braking performance of an aircraft on a runway the to be contaminated, and hence the required stopping distance, is difficult to predict due to the difficulty in having reliable and precise knowledge of the runway state, which is essential to the deceleration of the aircraft.
Traditionally, the runway state is determined by ground crew, or evaluated by a pilot during the landing and delivered in a landing report. This information on the runway state, transmitted to aircraft on approach, is nonetheless rather unreliable and potentially becomes outdated quite quickly. Specifically, runway state characteristics have a high degree of time volatility.
In order to make the estimation of a runway state reliable, documents FR2930669 and FR2978736 propose solutions making it possible to automatically estimate the landing runway state on the basis of measured levels of braking performance of an aircraft during its landing, regardless of the type of aircraft.
However, the runway state thus determined and delivered to aircraft on approach does not allow a potential deterioration in the runway occurring between the two landings to be taken into account.
In order to take this potential deterioration in the runway into account, document FR3007179 envisions determining local information depending on a local runway state characterizing an area of runway on which the aircraft is in movement during the landing. This local information, when it indicates that a local runway state is worse than a reference runway state, is used to update, in real time or near real time, the runway state or a braking datum resulting therefrom.
The updated braking datum may then be delivered as input to a brake assist module, which, in response, generates a braking setpoint for controlling a brake device of the aircraft.
The process of updating the runway state is referred to as a “unidirectional” process since only a downgrading thereof is permitted, without the possibility of upgrading it during the landing. This limitation has been put in place for safety reasons. Specifically, a temporary improvement in the deceleration capabilities of the aircraft due to an upgraded assessment of the runway state should not be banked on, since nothing guarantees that this upgraded state will last until the aircraft comes to a stop.
FIG. 1, taken from the document FR3007179, illustrates a system for assisting the piloting of an aircraft in landing phase according to this same document. In this system, the determination of whether the local runway state is worse than the reference runway state is carried out by comparing two data of the same nature, iref and iloc. These items of reference and local information are either runway states or current levels of braking or deceleration of the aircraft.
However, a drawback of this system based on braking or deceleration levels is that it applies the same criteria triggering the updating of the runway state or the braking datum throughout landing and movement on the runway.
The present disclosure aims to improve the pilot assist of an aircraft, in particular in the landing phase.